Towards stable polymer solar cells through crosslinking and sidechain cleavage

This thesis focuses on the investigation of two strategies for stabilizing the photovoltaic performance of polymer solar cells. Polymer solar cells (PSC) fabricated using solution processable conjugated polymers offer the potential for high-speed production of solar cell modules at low cost. However, achieving high power conversion efficiency (PCE) and long lifetime remains a challenge for PSCs. Photoexcitation of a conjugated polymer generates tightly bound electron-hole pairs (i.e., excitons) that require an electron acceptor (e.g., fullerene) for the dissociation of excitons into free charges. An interpenetrating network of conjugated polymer and fullerene (i.e., bulk heterojunction) is ideal for efficient charge generation due to the existence of a high interfacial area. However, such a morphology is not thermodynamically stable and is subject to large phase segregation in the form of fullerene aggregation provoked by the build-up of excessive heat during the operation of PSCs. Consequently, the PCE of PSCs degrades over time. In this thesis, two strategies for morphological stabilization are investigated using derivatives of poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-b]thiophene) (PTB). In one strategy, a derivative of PTB having thermally-cleavable tetrahydropyran (THP) sidechains (PTB(THP)) is synthesized. Removal of the THP sidechains by thermal annealing reduces the mobility of PTB, thus retarding the diffusion of fullerene through the polymer matrix to form large aggregates. Photovoltaic (PV) devices made from PTB(THP) after thermal-cleavage of the sidechains exhibited stable PCE over prolonged thermal annealing, which is attributed to the thermally-stable morphology observed by microscopic studies. In the second strategy, a series of PTB derivatives bearing photocrosslinkable chlorooctyl sidechains (PTB-Cl) are synthesized. Photocrosslinking initiated by deep UV is able to insolubilize thin films of PTB-Cls. PV devices having stable PCE over prolonged thermal annealing were demonstrated using photocrosslinked PTB-Cls. However, high-number of photocrosslinkable sidechains and prolonged UV irradiation pose negative effects on the PCE and stability of PV devices made from PTB-Cls. In addition, accurate PCE measurement is of importance for the research of PSC. In Chapter 4, measurement errors of PCE using a simple xenon arc lamp are discussed. Improvements of measurement accuracy are demonstrated following simple modifications of the instruments and the measurement procedure.